Q: Weight loss is based on expending more energy than consumed, and for every 3,000 calorie deficit you lose one pound. Assuming both an obese person and someone slightly overweight eat proportionate calories and exercise at equal levels, why does the obese person lose a lot more weight in the same amount of time than the slightly overweight person?
A. There are two reasons for this discrepancy. The first is, what weight are you losing? Contrary to popular belief, the first pounds one loses in response to dieting are water. Caloric restriction initially results in a loss of the glucose storage polymer known as glycogen from liver and muscle.
Glycogen, because of its tree-like structure and polar (electrically charged) nature, is surrounded by lots of water molecules. When you chew up glycogen for energy, the water get liberated and you urinate it out. Obese people have more glycogen to lose than merely overweight people, so more water weight is lost in the first few days.
The second reason is that the obese person burns more calories at baseline. At baseline, everyone’s resting energy expenditure is 50 kcal/kg fat-free mass. While the obese person has more fat mass, they also have more fat-free (muscle and bone) mass. So if they restrict their calories, since they are burning more, they will therefore lose more energy in the same period of time. However, as they do, the hormone leptin (which comes from the fat cells) drops, and the brain interprets that drop as a starvation signal, and reduces resting energy expenditure to as little as 42 kcal/kg fat-free mass, thus preventing them from losing a whole lot more. That’s why obese people tend to plateau their weight within two weeks of dieting.
Robert H. Lustig, MD, UC San Francisco Children's Hospital is a pediatric endocrinologist and
director of the Weight Assessment for Teen and Child Health clinic.
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